Electrolytic treating apparatus

ABSTRACT

An electrolytic treating apparatus employing the principle of high-current density bi-polar electrolyzing, wherein the current is conducted from the electrodes through the electrolyte to the metal strip being treated. The efficiency of the resulting electrolyzing treatment is enhanced by employing (a) electrodes of a generally circular cross-section and (b) offset electrode positioning, so that the electrodes facing the top surface of the strip do not overlie the electrodes facing the bottom surface of the strip.

United States Patent Brendlinger et al.

[ Dec. 16, 1975 ELECTROLYTIC TREATING APPARATUS Edward C. Brendlinger, Pitcairn; Richard F. Higgs, Monroeville; Issa J. Kharouf, Pittsburgh, all of Pa.

United States Steel Corporation, Pittsburgh, Pa.

Filed: Jan. 21, 1975 Appl. No.: 542,757

inventors:

Assignee:

US. Cl 204/206; 204/28; 204/145 R; 204/207; 204/211; 204/268 Int. Cl. C25F 7/00 Field of Search 204/28, 145 R, 206, 207, 204/211, 268, DIG. 7

References Cited UNlTED STATES PATENTS 7/1939 Yerger et al 204/211 Primary Examiner-John H. Mack Assistant Examiner-W. 1. Solomon Attorney, Agent, or Firm-Arthur J. Greif 7 Claims, 1 Drawing Figure ELECTROLYTIC TREATING APPARATUS This invention relates to an apparatus for the high current density electrotreating (pickling. cleaning. etc.) of metal strip and is more particularly related to improvements in electrode positioning and construction.

Virtually all production of continuous lengths of metal strip results in undesirable residues remaining on the surface of the strip. Such residues may, for example, be oxides which are generally removed by pickling in acid, or lubricants which are removed by cleaning in alkaline detergents. When the end uses of the strip require a particularly clean surface (eg. when the metal is to be coated as in tin-plate) the art often resorts to electrolytic processes which, in general, provide for more effective residue removal. Presently, the electrolytic treatment of continuously moving metal strip is accomplished by passing the strip between electrodes having a generally rectangular cross-section. Current transfer to the strip is achieved by either of two basic means:

1. use of conductor rolls, etc. for making direct elec' trical contact with the strip, or

2. transfer of current flow from an electrode of one polarity, through the electrolyte to the strip and again through the electrolyte to an electrode of opposite polarity. Since the danger of sparking at contact points between the strip and rolls increases as the current density employed increases, the latter method, which is sometimes referred to as bi-polar electrolyzing, has been found especially beneficial (see, for example, US. Pat. No. 3,338,809) for electrolytic treatments employing very high current densities. Although not directed to the use of very high densities, US. Pat. No. 2,165,326, shows an electrotreating cell which may be employed for effecting bi-polar electrolyzing. However, if such a cell is employed for the electrolytic treatment of metal strip at very high current densities, i.e. in excess of about 500 Amps/ft and more desirably within the range 2,000 10,000 Amps/ft it is found that current efficiency is markedly decreased as a result of large losses within the electrolyte itself. Additionally, it has been found, even when employing bi-polar electrolyzing, that such very high current densities nevertheless often give rise to arcing and other problems associated with stray currents, such as overheated bearings.

It is therefore a principal object of this invention to provide an apparatus useful for the bi-polar electrolyzing of metal strip.

It is further object to provide a bi-polar electrolyzing apparatus which is capable of achieving significantly enhanced current efficiencies.

It is yet another object of this invention to provide a bi-polar electrolyzing apparatus utilizing particular electrode configurations and connections for minimizing the deleterious effects of stray currents.

These and other objects and advantages of the present invention will be more apparent from the following description when taken in conjunction with the appended claims and drawing, in which The FIGURE is a diagrammatic representation of a longitudinal cross-section of the apparatus, illustrating the essential features of this invention.

Referring to the FIGURE, the apparatus consists of a tank 2, containing an electrolyte (not shown) suitable for the contemplated electrolytic treatment. The metal strip 3, is passed in a substantially horizontal plane, through non-conductive bumper rolls 4a, into the tank and between electrode pair configurations 5a 5b, 6a 6b, and 7a 7b. respectively. The electrodes are oriented in a conventional electrotreating position; that is. their longitudinal axes (1') lie in a segment of a plane which is parallel to the plane segment formed by the strip face and (ii) are generally perpendicular to the direction of strip travel. On exiting the tank, the strip is again passed through bumper rolls 4b, placed somewhat closer to the strip than the electrodes themselves. Such bumper rolls, which also act as guide rolls, serve to prevent undesirable contact of the strip with the electrodes. Their use is especially desirable in commercial practice, where a high rate of strip travel (increased vibration) and poor strip shape (waviness) would both combine to make such undesirable contact a virtual certainty. Plastic pipe bumpers 8 may also be placed within the tank to provide additional insurance against such undesirable contact. In addition to the use of bumpers, however, it is a feature of this invention to preferably employ an odd number n, of electrode pair configurations. Three such pair configurations are illustrated, but it should be understood that 5, 7, etc. such configurations may be employed, as well. In using such an odd number of electrode pair configurations, it has been found that the deleterious effects of stray currents are to a substantial extent eliminated, thereby further reducing the tendency to arcing and/or strip burns, attendant in the use of very high current densities. In employing an odd number of electrode pair configurations, it is desirable that the cross-sectional area of the positively charged electrodes be approximately equal to that of the negatively charged electrodes. This factor is illustrated in the figure, wherein the negatively charged electrodes 6a 6b, are of a greater cross-section and are therefore each capable of conducting a greater amount of current than any of the individual positively charged electrodes.

Three features for improving current efficiency are also shown in the FIGURE:

a. In contrast with electrodes conventionally employed in treating strip, it is essential that the portion of the electrodes facing the top surface of the strip exhibit a convex shape and preferably be of a generally semicircular shape. For ease of construction, it will generally be most practical to produce cylindrical electrodes, i.e. electrodes with a circular cross-section, as shown in the FIGURE. It should be understood, however, that the portion of the electrodes not facing the strip, i.e., the top portions of electrodes 5a, 6a and 7a may, for example, be flat. Similarly, the shape of the bottom electrodes is not critical and conventional electrodes of rectangular cross-section may be employed. In actual practice a large number of electrodes will be produced at one time. In order that the electrodes may be interchangeable it will generally be preferable to employ bottom electrodes 5b, 6b, 7b which are similarly of a generally cylindrical shape.

The enhanced current efficiency, resulting from the use of top electrodes of circular cross-section, is shown in Table I. Ferrous metal strip having an oxide coating thereon, was electro-pickled in 20% H 50 solution maintained at a temperature of l25F. The distance from both the top and bottom electrodes, to the strip; was 1.5 inches, for all runs. The strip was passed at tliE same speed in all cases to provide an 'electrolyziiiig period of three seconds. The data is ad average of foiil Table I High Current Density Pickling Efficiency Electrodes of rectangular vs. circular cross-section Pickling Efficiency-7r Current Rectangular Circular Efficiency Density-Amps/ft Electrode Electrode Increase-K b. It is seen, with respect to any electrode pair configuration; for example 5a-5b, that contrary to conventional placements, top electrode 5a does not overlie electrode 5b. It has been found that additional enhancement of efficiency may be achieved if the electrodes forming a pair configuration do not so overlie one another. Stated another way, if we imagine a vertical axis drawn through both the top and bottom electrodes, then the distance between these two axes should be greater than the sum of the cross-sectional widths of both electrodes, divided by 2. In the specific case where round electrodes are employed, then the distance between the vertical axes should be greater than: the sum of the diameters of both electrodes divided by 2 (i.e. greater than the sum of the two radii). The beneficial effect of such electrode offset is shown in Table II. Unless otherwise stated, electrolyzing conditions were the same as above. However, utilizing the knowledge gleaned from the data of Table I, only round electrodes were evaluated in the runs below.

Table II High Current Density Pickling Efficiency Ovcrlying Electrodes vs. Offset Electrodes Pickling Efficiency-% c. Separating the respective electrode pair configurations are insulative baffles 9 for directing the current through the strip. These bafiles, which may, for example, be made of polypropylene, are positioned a short distance from the strip surface, but nevertheless sufficient so that the passage of the strip therethrough is not encumbered. Since a significant portion of the current would tend to travel in a straight line between the electrodes (eg. a line between 5a and 6a) it may be seen that the baffles serve to block that route, and force that otherwise lost current through the strip. It is therefore desirable that the baffles be sufficiently close to the strip so as to cause a substantial portion of the current which would normally travel in such straight lines, to be diverted therefrom and travel through the strip (see the dashed lines of the FIGURE).

In addition to the above noted features for improving current efficiency, it is nevertheless desirable that the effects of concentration polarization be reduced by employing any of the well known techniques for effecting stirring of the solution. For example, simple mechanical or propellor-type stirrers may be employed. Particularly good results have been achieved through the use of flow headers 10 (driven by a circulation pump not shown) which force a low pressure jet of electrolyte toward the strip face at a small angle, eg. 30. Even when current densities within the range 2000 10,000 Amps/ft are employed, it has been found that stirring of the electrolyte need not be particularly turbulent, especially if the flow headers are strategically positioned, as shown, near those portions of the strip surfaces undergoing maximum electrochemical activity.

We claim:

1. An apparatus for the electrolytic treatment of metal strip within an electrolyte bath, comprising:

a. means for guiding said strip in a substantially horizontal plane, through said bath,

b. a number, n, of electrode pair configurations, n being at least 2, in which the electrodes forming each of said pairs are,

i. in an electrotreating position, with one electrode of said pair above said substantially horizontal plane and the other electrode of said pair below said substantially horizontal plane,

ii. offset from each other so that the distance between their respective vertical cross-sectional axes is greater than the sum of their cross-sectional widths/2, and

iii. the portion of those electrodes facing the top surface of the strip, exhibits a convex shape with respect to said top surface;

c. a d-c source of EMF, adapted to supply a current density within the range of 500 to 10,000 Amps/ft to said strip, and connected so as to make the electrodes in each pair approximately the same potential and of a polarity opposite to that of the pair configurations adjacent thereto; and

d. separating each said pair configuration, insulative baffles positioned for the unencumbered passage of said strip, whereby the major portion of the current passing between oppositely charged electrodes is caused to flow through the strip.

2. The apparatus of claim 1, in which said electrode portions exhibiting a convex cross-section are substantially semicircular, and said d-c source of EMF is adapted to supply a current density of at least 2000 Amps/ft? 3. The apparatus of claim 2, in which n is an odd number.

4. The apparatus of claim 3, in which the electrodes above said substantially horizontal plane are of a generally cylindrical shape.

5. The apparatus of claim 4, in which all the electrodes are of a generally cylindrical shape.

6. The apparatus of claim 5, in which the cross-sectional area of all positively charged electrodes is approximately equal to the cross-sectional area of all negatively charged electrodes.

7. The apparatus of claim 6, including electrolyte flow headers for increasing solution turbulence near those portions of the strip surfaces undergoing maximum electrochemical activity, whereby concentration polarization is decreased at least to extent sufficient to support current densities within said range. 

1. AN APPARATUS FOR THE ELECTROLYTIC TREATMENT OF METAL STRIP WITHIN AN ELECTROLYTE BATH, COMPRISING: A. MEANS FOR GUIDING SAID STRIP IN A SUBSTANTIALLY HORIZONTAL PLANE, THROUGH SAID BATH, B. A NUMBER, N, OF ELECTRODE PAIR CONFIGURATIONS, N BEING AT LEAST 2, IN WHICH THE ELECTRODES FORMING EACH OF SAID PAIRS ARE, I. IN AN ELECTROTREATING POSITION, WITH ONE ELECTRODE OF SAID PAIR ABOVE SAID SUBSTANTIALLY HORIZONTAL PLANE AND THE OTHER ELECTRODE OF SAID PAIR BELOW SAID SUBSTANTIALLY HORIZONTAL PLANE, II. OFFSET FROM EACH OTHER SO THAT THE DISTANCE BETWEEN THEIR THE SUM OF THEIR CROSS-SECTIONAL WIDTHS/2, AND THAN THE SUM OF THEIR CROSS-SECTIONAL L WIDTHS/2, AND III. THE PORTION OF THOSE ELECTRODES FACING THE TOP SURFACE OF THE STRIP, EXHIBITS A CONVEX SHAPE WITH RESPECT TO SAID TOP SURFACE;
 2. The apparatus of claim 1, in which said electrode portions exhibiting a convex cross-section are substantially semicircular, and said d-c source of EMF is adapted to supply a current density of at least 2000 Amps/ft2.
 3. The apparatus of claim 2, in which n is an odd number.
 4. The apparatus of claim 3, in which the electrodes above said substantially horizontal plane are of a generally cylindrical shape.
 5. The apparatus of claim 4, in which all the electrodes are of a generally cylindrical shape.
 6. The apparatus of claim 5, in which the cross-sectional area of all positively charged electrodes is approximately equal to the cross-sectional area of all negatively charged electrodes.
 7. The apparatus of claim 6, including electrolyte flow headers for increasing solution turbulence near those portions of the strip surfaces undergoing maximum electrochemical activity, whereby concentration polarization is decreased at least to extent sufficient to support current densities within said range. 